The present invention relates to registration of plural image exposures formed on a photoreceptor belt in a single pass by a plurality of LED print bars and, more particularly, to improvements in lateral registration by use of high resolution print bars.
Image print bars used in xerographic recording systems are well known in the art. The print bar generally consists of a linear array of a plurality of discrete light emitting sources. Light emitting diode (LED) arrays are preferred for many recording applications. In order to achieve high resolution, a large number of light emitting diodes, or pixels, are arranged in a linear array on a common substrate. The orientation of the print bar is transverse to the movement of the photoreceptor (transverse to the process direction). Each LED in a central, or image forming area of the array is used to expose a corresponding area on a moving photoreceptor to an exposure value defined by the video data information applied to the drive circuits of the print bars. The photoreceptor is advanced in the process direction to provide a desired image by the formation of sequential scan lines.
In one embodiment of a color xerographic printer, a plurality of LED print bars are positioned adjacent to a photoreceptor belt surface and selectively energized to create successive image exposures. If two bars are used, the system typically provides one highlight color and one black color. For full color, four bars are used, one for each of the three basic colors and a fourth print bar for black images.
When a full color system is implemented, each color image must be precisely aligned (registered) so that all corresponding pixels in each image area are registered. Current requirements are for registration tolerance of approximately 100.mu.. This is a cumulative total which includes 25.mu. for lateral motion of the photoreceptor, 25.mu. for the skew motion of the photoreceptor, and the remainder for the photoreceptor motion quality in the process direction and lateral belt motion (cross process direction). The present invention is directed towards a method and apparatus for improved registration of the print bar areas in the lateral or cross process direction.
There are several prior art techniques for registering multiple print bars in the lateral direction. Co-pending application U.S. Ser. No. 07/807,931, assigned to the same assignee as the present invention, discloses LED print bar registration techniques which utilize hole sensors to generate registration correction signals which are used to drive stepper motors which provide transverse and rotational motion to an associated image bar. Another U.S. Pat. No. 5,208,796, assigned to the same assignee as the present invention, discloses lateral registration techniques which include physical shifting of the print bar by operation of piezoelectric stacks. A still further co-pending application U.S. Ser. No. 07/862,150, assigned to the same assignee as the present invention, discloses lateral registration of print bars by use of pairs of oppositely aligned holes formed on the photoreceptor, beneath which are placed sensors which detect light from selected print bar pixels. The contents of these three applications are hereby incorporated by reference.
The present invention is directed towards an improved technique for obtaining lateral registration of multiple print bars in a single pass printer, which does not require mechanisms for lateral movement of the print bars to obtain registration nor the use of hole pairs on opposite sides of the photoreceptor belt. The invention isolates two tolerances, the pixel position and the lateral motion components (collective total of 50.mu.) and provides a print bar control system which establishes an initial registration for each print bar that places it within the 50.mu. tolerance, and then maintains print bar operation within that tolerance range by detecting subsequent lateral motion of the print bar and/or the photoreceptor and generate corrective signals within a control system to maintain the registration within the prescribed tolerances. More particularly, the present invention relates to a method for lateral registration of sequential images formed in a single pass on the surface of a photoreceptor belt moving in a process direction including the steps of:
(a) positioning a plurality of light emitting diode (LED) print bars in image-forming relation with respect to said belt, said print bar having a first linear segment of light emitting pixels which are selectively addressed to form said sequential images, said print bar having a second, non-image forming segment of light emitting pixels at one end contiguous to said image-forming segment, PA0 (b) forming a Z-shaped pattern at one end of the photoreceptor belt, said pattern comprising a first, second and third angled slot aperture aligned in the process direction, PA0 (c) moving the photoreceptor so that light from the non-image forming pixels is periodically visible through said slot apertures, as said belt moves the apertures past said print bars, PA0 (d) selectively energizing at least one of said non-image forming pixels of a first print bar, PA0 (e) detecting the passage of light through said apertures, PA0 (f) measuring the time between detection of the light passing through the first and second apertures passing beneath the instant energized pixel to establish a time t.sub.1, PA0 (g) measuring the time between detection of the second and third apertures passing beneath the instant energized pixel to establish a second time t.sub.2, PA0 (h) continuing selective energization of pixels in said end array unit t.sub.1 =t.sub.2, thereby establishing the instant energized pixel as a fiduciary pixel, PA0 (i) establishing a #1 scan pixel in said first linear segment, PA0 (j) repeating steps (d) through (i) for the second, third and fourth print bars, PA0 (k) establishing a tolerance value whereby t.sub.1 is considered equal to t.sub.2 in said step (h), PA0 (l) periodically repeating steps (d), (g) and (h) to detect a condition where t.sub.1 is no longer equal to t.sub.2 within the established tolerance for at least one print bar, and PA0 (m) establishing a new fiduciary pixel and a new start of scan pixel to return to the condition of t.sub.1 =t.sub.2 within the prescribed tolerance.